As used herein, a heat-generating composition has the meaning of a powder or paste composition as an intermediate product. A heater has the meaning of a formed and sintered product made from the intermediate product.
This invention relates generally to a heat-generating composition, a heater, and more particuraly to a ceramic heater comprised of the heat-generating composition which is suitable for use in the home or in industry, because of having excellent heat-resistance and low electrical-resistance properties.
Japanese Laid-Open Patent Publication No. 53-50496 and No. 53-59897 disclose resistor compositions which comprise mixtures selected from electrically conductive substances such as, for example, molybdenum silicide, tungsten silicide, tantalum silicide, magnesium silicide, manganese silicide, with a glass frit. The resistor composition is prepared by mixing the components in a specified mixing ratio.
According to the foregoing prior art, the softening point of the glass frit is from 600.degree. to 700.degree. C. When glass frit is used as a binder for printing a surface heater pattern on a substrate, normal processing requires sintering at a temperature of from 900.degree. to 1,000.degree. C. Such temperatures are so far above the softening point of the glass frit that a printed pattern of the heat-generating composition may collapse. In this case, the resulting heater may be short-circuited. Further, if such glass frit is used to form a rod-shaped heater, the heat generated during use at a high temperature may deform the rod.
U.S. Pat. No. 4,597,897 discloses a hexaboride resistor composition which comprises conductive hexaboride particles and a non-reducible glass. The glass may be either crystalline or noncrystalline, and contains 20-35 mol % B.sub.2 O.sub.3 and 10-30 mol % M.sup.II O (wherein M.sup.II is Ca, Sr, or Ba). However, the glass also contains 35-55 mol % SiO.sub.2. Therefore, this prior art only discloses a resistor, and the properties thereof are completely different from those of a heater.
Japanese Laid-Open Patent Publication No. 57-128003 discloses a metallized composition used for forming a heat-generating pattern. The composition contains a selected weight ratio of tungsten to molybdenum. This material has a high temperature coefficient of resistance.
Japanese Laid-Open Patent Publication No. 53-50498 discloses a process for producing a resistor composition of molybdenum silicide, in which the temperature coefficient of resistance is relatively low. However, the sheet resistively of the resistor or heater formed from the composition by sintering is high.
U.S. Pat. No. 4,985,377 discloses a glaze resistor in which both metal silicide and metal boride are used as electrically conductive substances. U.S. Pat. Nos. 4,119,573 and 4,323,484 also disclose glaze resistors in which metal silicides are used as electrically conductive substances. However, the purpose and usage of these resistors are different from that of a heater as a conductor. Accordingly, the ratio of the electrically conductive substances to the glass frit is naturally different between resistors and heaters, and also the electrical-resistance values of both differ greatly. Furthermore, the kind of additives and their effects are also different between resistors and heaters. In these resistors, additives increase the electrical-resistance and the stability of electrical-resistance. Further, above-mentioned U.S. Pat. Nos. 4,119,573 and 4,323,484 disclose glaze resistor compositions contained silicides, which can be sintered at a low temperature such as 850.degree. C. in air. However, resulting resistors have high electrical-resistances.
U.S. Pat. No. 4,882,212 discloses a ceramic-glass-metal composite material for a semiconductor casing. This composite material is an insulator which is completely different from a heater in characteristics and usage.
According to the conventional method of making a patterned ceramic heater, an electrically conductive powder and a glass frit are mixed, and a vehicle is added to produce a paste. The paste is printed in the desired pattern by a screen printing process. Then it is dried and sintered to make a ceramic heater. The ceramic heater prefers to have both a low electrical-resistance value and a low temperature coefficient of resistance (TCR) to perform effectively.
When the electrical-resistance value is low, the heater can be thinner, and also an arbitrary pattern for the uniform heat generation of the ceramic heater can be formed.
When the temperature coefficient of resistance (TCR) is low, it discreases a difference between an initial current value, immediately after switching on the heater, and a regular current value at a high temperature, during use of the heater. Thus, since the difference between the current value supplied to the heater immediately after energizing and the regular current value is smaller, it is suitable for use in the home. On the contrary, when the TCR value is large, the use of larger peripheral appliances is necessary. Additionally, when the TCR value is low, the heater generates heat uniformly.
Japanese Laid-Open Patent Publication No. 4-96201 discloses a heat-generating composition comprised a glass frit, an electrically conductive substance, and an additive. It is necessary that this composition is sintered at a high temperature such as 1,050.degree. C. in a nitrogen atmosphere. In the mean time, Japanese Laid-Open Patent Publication No. 4-96201 corresponds to Japanese Patent Application No. 2-207188 from which Paris Convention Priority is claimed for this invention.